Reflectance and Fluorescence imaging are used in numerous medical and research applications. By way of example only, White light endoscopy (WLE) is the standard approach for colon cancer screening. However, traditional WLE relies on native tissue contrast (reflectance), and lacks specificity. Autofluorescence imaging (AFI) and narrow-band imaging (NBI) have been applied in an effort to increase the ability to detect cancers of the colon. These approaches have, in some cases, shown increased sensitivity and specificity. However, various large-scale studies have shown negligible improvements over WLE. The low specificity is largely due to insufficient information in the one or two wavelength bands acquired. Accordingly, it is simply not possible to detect changes in the fluorescence associated with many biomarkers in the presence of autofluorescence from healthy tissue using AFI or NBI.
Previous studies have demonstrated that tumors have reflectance and/or fluorescence spectra that are different from surrounding tissue, and that sampling this spectrum can result in increased sensitivity and specificity. However, heretofore, there has been no suitable hyperspectral illumination device providing illumination with multiple, discrete narrow wavelength bands over a wide spectral range for practical use in reflectance or fluorescence imaging of multiple biomarkers.
Briefly, fluorescence is a chemical process wherein light of a specific wavelength shined upon a fluorescent molecule causes electrons to be excited to a high energy state in a process known as excitation. These electrons remain briefly in this high energy state, for roughly a nanosecond, before dropping back to a low energy state and emitting light of a lower wavelength. This process is referred to as fluorescent emission, or alternatively as fluorescence.
In a typical fluorescence imaging application, one or more types of fluorescent materials or molecules (sometimes referred to as fluorescent dyes) are used, along with an illuminator apparatus that provides the exciting wavelength, or wavelengths. Different fluorescent molecules can be selected to have visually different emission spectra. Since different fluorescent molecules typically have different excitation wavelengths, they can be selectively excited so long as the bandwidth of the excitation light for one fluorescent molecule does not overlap the excitation wavelengths of other fluorescent molecules that are present in the body being imaged. Therefore the excitation light should ideally have well defined bandwidths. Moreover, it may be desirable to use an intense light so as to increase the chances of the fluorescence process occurring.
Traditional fluorescence illuminators have relied on metal halide arc lamp bulbs such as Xenon or Mercury bulbs, as light sources. The broad wavelength spectrum produced by these lamps when combined with specific color or band pass filters allows for the selection of different illumination wavelengths. However, this wavelength selection and light shaping process is highly energy inefficient. In this regard, selecting only a relatively small portion of the wavelength spectrum produced by the Xenon or Mercury bulb results in the vast majority of the light output from the lamp being unused. Moreover, the wavelength selection or band pass filters are costly, especially when placed on a mechanical rotating wheel in typical multiple-wavelength applications.
When using metal halide arc lamp bulbs, the speed with which different wavelengths can be selected is limited by the mechanical motion of moving various filters into place. In addition to the sluggishness and unreliability of filter wheels, as well as energy coupling inefficiency, metal halide arc lamps are also hampered by the limited lifetime of the bulb. The intensity of the light output declines with bulb use and once exhausted, the user has to undergo a complicated and expensive process of replacing the bulb and subsequently realigning the optics without any guarantee that the illuminator will perform as before. These disadvantages make acquiring consistent results difficult and inconvenient for users who must deal with the variable output of the bulbs, and who must either be trained in optical alignment or call upon professionals when a bulb needs to be replaced.
A light-emitting diode (LED) is a solid state, semiconductor based light source. Modern LEDs are available to provide discrete emission wavelengths ranging from ultraviolet (UV) to infrared (IR). The use of LEDs as light sources overcomes numerous limitations of metal halide arc lamps. By way of example only, the lifetime of an LED is typically rated at well over 10,000 hours which is much greater than that of metal halide arc lamps. Moreover, the power output varies negligibly over the full life of the LED. In addition, the bandwidth of the spectral output of an LED chip is typically narrow (<30 nm) which can reduce or eliminate the need for additional band pass filters in a fluorescence application. Moreover, the intensity of the output light from an LED can be quickly and accurately controlled electronically by varying the current through the LED chip(s), whereas in metal halide illuminators, the output intensity of the bulb is constant and apertures or neutral density filters are used to attenuate the light entering the microscopy.
In the past, the number of LED light sources which could be aligned was limited to about 4 or 5 due to the relatively long optical paths required to combine the beams from the multiple chips or modules which are spatially separated using free space optics. This alignment difficulty has substantially limited the application of LED light sources in imaging applications since the desired high intensity for such applications is difficult to achieve. Attempts to address this deficiency have incorporated the use of additional optical elements such as lenses, mirrors and the like for each wavelength. However, the use of such optical elements has practical limitations due to their negative impact on intensity and uniformity of the treated light beams. These issues have limited the practical use of LED-based illuminators in reflectance and fluorescence imaging applications which require light that is both intense and spatially uniform.
Accordingly, there is a continuing need for an illumination device adapted to efficiently align light output from multiple wide band or narrow-band illumination sources such as LEDs, lasing diodes, or the like for delivery of high intensity, spatially uniform illumination to a field of observation. By way of example only, and not limitation, such an illumination device may be used in a hyperspectral reflectance or fluorescence imaging endoscope or microscope that can reveal pathology specific changes in the structure and molecular composition of tissues, allowing early detection and differentiation of pathological processes in the colon or other tissues.